A multi-channel orbital angular momentum (OAM) mode generation and switching scheme is proposed and demonstrated based on an in-fiber mode selective interferometer (MSI), which is formed in a four-mode fiber. The MSI consists of two strong modulated long-period fiber gratings (LPFGs), which realizes the mode selected coupling between a target mode pair. With the optimized structural parameters, the MSI can couple a launched LP01 (or OAM0) into a desired higher-order azimuthal mode (HAM, LPl1 or OAM±l, l≥1) at multiple wavelength channels and generate the HAM with high-purity. To verify this concept, we fabricate two LPFGs in a four-mode fiber with designed distance and hence realize a MSI which can generate the second-order HAM (OAM2 or LP21) at 17 wavelength channels. The mode conversion efficiency is more than 90% at 17 wavelengths and the corresponding mode purity is no less than 97%, respectively. In addition, we also demonstrate that the selected mode pair (OAM0 and OAM2) can be switched at multiple channels by changing the state of the MSI. This MSI can also be used as a wavelength band-rejection filter on different spatial modes and find potential applications in optical communications and sensing.